Dhcp Client Request Process - Cisco Catalyst 9400 System Management Configuration Manual
Cisco ios xe bengaluru 17.4.x
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Also See for Catalyst 9400:
- Hardware installation manual (192 pages) ,
- Installation note (32 pages) ,
- Troubleshooting manual (14 pages)
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Performing Device Setup Configuration
DHCP Client Request Process
Note
We recommend a redundant connection between a switch stack and the DHCP, DNS, and TFTP servers. This
is to help ensure that these servers remain accessible in case one of the connected stack members is removed
from the switch stack.
The DHCP server for your device can be on the same LAN or on a different LAN than the device. If the
DHCP server is running on a different LAN, you should configure a DHCP relay device between your device
and the DHCP server. A relay device forwards broadcast traffic between two directly connected LANs. A
router does not forward broadcast packets, but it forwards packets based on the destination IP address in the
received packet.
DHCP-based autoconfiguration replaces the BOOTP client functionality on your device.
DHCP Client Request Process
When you boot up your device, the DHCP client is invoked and requests configuration information from a
DHCP server when the configuration file is not present on the device. If the configuration file is present and
the configuration includes the ip address dhcp interface configuration command on specific routed interfaces,
the DHCP client is invoked and requests the IP address information for those interfaces.
This is the sequence of messages that are exchanged between the DHCP client and the DHCP server.
Figure 4: DHCP Client and Server Message Exchange
The client, Device A, broadcasts a DHCPDISCOVER message to locate a DHCP server. The DHCP server
offers configuration parameters (such as an IP address, subnet mask, gateway IP address, DNS IP address, a
lease for the IP address, and so forth) to the client in a DHCPOFFER unicast message.
In a DHCPREQUEST broadcast message, the client returns a formal request for the offered configuration
information to the DHCP server. The formal request is broadcast so that all other DHCP servers that received
the DHCPDISCOVER broadcast message from the client can reclaim the IP addresses that they offered to
the client.
The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPACK
unicast message to the client. With this message, the client and server are bound, and the client uses
configuration information received from the server. The amount of information the device receives depends
on how you configure the DHCP server.
If the configuration parameters sent to the client in the DHCPOFFER unicast message are invalid (a
configuration error exists), the client returns a DHCPDECLINE broadcast message to the DHCP server.
The DHCP server sends the client a DHCPNAK denial broadcast message, which means that the offered
configuration parameters have not been assigned, that an error has occurred during the negotiation of the
parameters, or that the client has been slow in responding to the DHCPOFFER message (the DHCP server
assigned the parameters to another client).
A DHCP client might receive offers from multiple DHCP or BOOTP servers and can accept any of the offers;
however, the client usually accepts the first offer it receives. The offer from the DHCP server is not a guarantee
that the IP address is allocated to the client; however, the server usually reserves the address until the client
System Management Configuration Guide, Cisco IOS XE Bengaluru 17.4.x (Catalyst 9400 Switches)
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Troubleshooting
